1. Field of the Invention
The invention in general relates to the fabrication of layered superlattice materials, and more particularly to fabrication processes that provide low fatigue ferroelectric and reliable high dielectric constant integrated circuit devices that are unusually resistant to degradation.
2. Statement of the Problem
Layered superlattice materials are a class of ferroelectrics and high dielectric constant materials discovered by G. A. Smolenskii, V. A. Isupov, and A. I. Agranovskaya. See Chapter 15 of the book, Ferrelectrics and Related Materials, ISSN 0275-9608, (V.3 of the series Ferroelectrics and Related Phenomena, 1984) edited by G. A. Smolenskii, especially sections 15.3-15.7; G. A. Smolenskii, A. I. Agranovskaya, "Dielectric Polarization of a Number of Complex Compounds", Fizika Tverdogo Tela, V. 1, No. 10, pp. 1562-1572 (October 1959); G. A. Smolenskii, A. I. Agranovskaya, V. A. Isupov, "New Ferroelectrics of Complex Composition", Soviet Physics - Technical Physics, 907-908 (1959); G. A. Smolenskii, V.A. Isupov, A. I. Agranovskaya, "Ferroelectrics of the Oxygen-Octahedral Type With Layered Structure", Soviet Physics - Solid State, V. 3, No. 3, pp. 651-655 (September 1961); E. C. Subbarao, "Ferroelectricity in Mixed Bismuth Oxides With Layer-Type Structure", J. Chem. Physics, V. 34, 695 (1961); E. C. Subbarao, "A Family of Ferroelectric Bismuth Compounds", J. Phys. Chem. Solids, V. 23, pp. 665-676 (1962) and Chapter 8 pages 241-292 and pages 624 & 625 of Appendix F of the Lines and Glass reference cited above.
As outlined in section 15.3 of the Smolenskii book, the layered perovskite-like materials can be classified under three general types:
(I) compounds having the formula A.sub.m-1 Bi.sub.2 M.sub.m O.sub.3m+3, where A=Bi.sup.3+, Ba.sup.2+, Sr.sup.2+, Ca.sup.2+, Pb.sup.2+, K.sup.+, Na .sup.+ and other ions of comparable size, and M=Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, Mo.sup.6+, W.sup.6+, Fe.sup.3+ and other ions that occupy oxygen octahedra; this group includes bismuth titanate, Bi.sub.4 Ti.sub.3 O.sub.12 ;
(II) compounds having the formula A.sub.m+1 M.sub.m O.sub.3m+1, including compounds such as strontium titanates Sr.sub.2 TiO.sub.4, Sr.sub.3 Ti.sub.2 O.sub.7 and Sr.sub.4 Ti.sub.3 O.sub.10 ; and
(III) compounds having the formula A.sub.m M.sub.m O.sub.3m+2, including compounds such as Sr.sub.2 Nb.sub.2 O.sub.7, La.sub.2 Ti.sub.2 O.sub.7, Sr.sub.5 TiNb.sub.4 O.sub.17, and Sr.sub.6 Ti.sub.2 Nb.sub.4 O.sub.20.
Up to now, these layered superlattice materials have not been considered as being suitable for non-volatile ferroelectric memories, nor have they been recognized as useful high dielectric constant materials. Attempts have been made to use two of the layered ferroelectric materials, bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12) and barium magnesium fluoride in a switching memory application as a gate on a transistor. See "A New Ferroelectric Memory Device, Metal-Ferroelectric Semiconductor Transistor", by Shu-Yau Wu, IEEE Transactions On Electron Devices, August 1974, pp. 499-504, which relates to the Bi.sub.4 Ti.sub.3 O.sub.12 device, and the article "Integrated Ferroelectrics" by J. F. Scott, C. A. Paz De Araujo, and L. D. McMillian in Condensed Matter News, Vol. 1, No. 3, 1992, pp. 16-20, which article is not prior art to this disclosure; However, Section IIB of the article discusses experiments with ferroelectric field effect transistors (FEFETs), the exact date of which experiments is not presently known to the inventors. Neither of these devices were successful. In the case of the Bi.sub.4 Ti.sub.3 O.sub.12 device, the ON state decayed logarithmically after only two hours, and in the case of the BaMgF.sub.4 device, both states decayed exponentially after a few minutes. See "Memory Retention And Switching Behavior Of Metal-Ferroelectric-Semiconductor Transistors", by S. Y. Wu, Ferroelectrics, 1976 Vol. 11, pp. 379-383.
It is believed that a key reason why none of the layered superlattice materials have been successfully used in a ferroelectric, high dielectric constant, or other practical application is that the processes of preparing the materials and fabricating devices from the materials that have been employed up to now have not been capable of producing high quality specimens of the materials. The materials used by Smolenskii and his collaborators were powdered materials in which the layered structure was broken and jumbled. The devices described in the papers by Wu cited above were films about 3 to 4 micrometers thick produced by sputtering. A recent paper, that is not believed to be prior art to the present disclosure, discloses the fabrication of a thin film of Bi.sub.4 Ti.sub.3 O.sub.12, a layered superlattice material, by spin coating sol-gel onto a substrate and annealing. Joshi, P. C. et al., "Structural and Optical Properties of Ferroelectric Thin Films By Sol-gel Technique," Appl. Phys. Lett., Vol 59, No. 10, November 91.
Prior to about 1988, the vast majority of ferroelectric thin films were made by either RF sputtering or chemical vapor deposition (CVD). See "Metalorganic Chemical Vapor Deposition of PbTiO.sub.3 Thin Films", by B. S. Kwak, E. P. Boyd, and A. Erbil, Applied Physics Letters, Vol. 53, No. 18, Oct. 31, 1988, pp. 1702-1704. About 1988, several other approaches began to be developed. The Kwak paper just cited discloses one such development, namely metalorganic chemical vapor deposition, or MOCVD.
The process of applying precursor solutions comprising organic compounds of the elements of the desired thin film in solution to substrates, and then heating the precursor on the substrate to form the film, has been used for many years in manufacturing and is known to be a simple process. However, until recently, all the manufactured products the process has been applied to have been large objects, such as windshields, in which the microscopic properties of the films have not been critical. It was generally thought that since an inherent part of the process is that significant quantities of organic materials must be disassociated from the elements of the future thin film and evaporated from the film as it dries, therefore the resulting film must have a relatively porous structure. About the same time as the MOCVD research mentioned above, the use of the precursor application/heating process was being developed as a process was to produce inks, such as for ink jet or screen printing, for forming conductors, resistors, and dielectric films on circuit boards or other substrates such as metal foil or glass. In this process, films of metal oxides were fabricated by dissolving a carboxylate of the metal in a solvent, such as xylene, spinning, dipping the a stoichiometric mixture of the solution on a substrate to form a thin film of liquid, and drying and firing the film to remove the solvents and organics and form the metal oxide thin film. See "Synthesis of Metallo-orgainic Compounds for MOD Powers and Films", G. M. Vest and S. Singaram, Materials Research Society Symposium Proceedings, Vol. 60, 1986 pp. 35-42. Specifically, this paper discloses the reaction of a metal compound, such as barium, bismuth, calcium, lead, strontium, zirconium, ruthenium, and tin compounds with either neodecanoic acid or 2-ethylhexanoic acid to form a metal carboxylate, such as bismuth 2-ethylhexanoate, which is dissolved in xylene to produce a precursor solution from which a thin film of a simple metal oxide, such as Bi.sub.2 O.sub.3, is produced. In one instance these processes were used to produce thin ferroelectric thin films of PbTiO.sub.3 (not a layered superlattice material), and since the processing conditions for making the films is compatible with silicon technology, it was postulated that it would be possible to use the process to make capacitors, detectors, and sensors on chips. See "PbTiO.sub.3 Thin Films From Metalloorganic Precursors", Robert W. Vest and Jiejie Xu, IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol 35, No. 6, November 1988, pp. 711-717. It is also known that these processes can be used to form thin films of other ceramics and superconductors. W. W. Davidson, S. G. Shyu, R. D. Roseman, and R. C. Buchanan, "Metal Oxide Films from Carboxylate Precursors", Materials Research Society Symposium Proceedings, Vol. 121, pp. 797-802 (1988). Concurrently with the development of the MOCVD and metal carboxylate processes for printing purpose summarized above, experiments were performed to explore the use of the same or similar precursors as used in the above process to produce ferroelectric thin films by spinning the precursor onto a substrate and drying it. The experiments showed the preferred precursors to be the metal alkoxides and acetates. See U.S. Pat. No. 5,028,455 issued to William D. Miller et al. and the article "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories", in Ferroelectrics, Vol 109, pp. 1-23 (1990). While the Miller patent mentions one metal carboxylate, lead tetra-ethylhexanoate, as a possible precursor, it was rejected as less desirable since the large organic group was thought to result in more defects in the final film. Further, the fact that the metal carboxylate processes were developed to produce inks such as for ink jet or screen printing, while the metal alkoxide and acetate processes were developed specifically for ferroelectric materials, also argued against the use of the metal carboxylates in ferroelectric integrated circuit applications.
In spite of the considerable research outlined above, none of the thin films made by the above processes, including the thin films of PbTiO.sub.3 made by Vest or the thin films of PZT made by Miller, produced reliable, low fatigue ferroelectrics, and in particular, none produced reliable, low fatigue layered superlattice thin films.
3. Solution to the problem:
The invention solves the above problems by providing a standardized process for making high quality thin films of a layered superlattice material. The microscopic quality of the film provides excellent properties in integrated circuits. The process is simple and compatible with conventional integrated circuit materials and processes.
The process starts with preparing a precursor solution containing each of the metals in the desired thin film compound. For most metals, a metal carboxylate with medium-length ligands is the preferred precursor compound. "Medium-length ligands" herein means ligands with between 6 and 9 carbon links. A metal 2-ethylhexanoate works for most metals. If the metal is titanium, the precursor may comprise titanium 2-methoxyethoxide having at least a portion of its 2-methoxyethoxide ligands replaced by 2-ethylhexanoate.
The precursor is dissolved in a solvent having a boiling point greater than 110.degree. C. A xylenes solvent works for most metals. For highly electropositive elements, the solvent includes 2-methoxyethanol or n-butyl acetate.
The precursor is applied to the integrated circuit wafer by spin coating or using a mist deposition process.
Optionally, the coated wafer is dried by heating it at about 150.degree. C. for a few minutes. It is then baked at about 250.degree. C. for from 30 seconds to a few minutes. The wafer is then annealed in an oxygen furnace at about 700.degree. C. for about an hour. Several, coating, drying, baking and annealing steps may be repeated for thicker films.
Alternatively, a sputtering target may be prepared in the above manner and the material deposited by sputtering it onto the integrated circuit.
If the metal is lead or bismuth, 1% to 75% excess metal is included in the precursor to account for evaporation of the oxide during baking and annealing. The best results were obtained for 50% excess bismuth. If sputtering is used, even higher excess amounts may be required for best results.
Extremely high quality thin films have been made by the above process. For example, strontium bismuth tantalate (SrBi.sub.2 Ta.sub.2 O.sub.9) has been made in thin films suitable for use in integrated circuits and having a polarizability, 2Pr, of 24 microcoulombs/cm.sup.2 and showing less than 5% fatigue after 10.sup.10 cycles, which is equivalent to 10 years use in a typical integrated circuit switching device. As another example, barium bismuth tantalate (BaBi.sub.2 Ta.sub.2 O.sub.9) having a dielectric constant of 166, measured at 1 Megahertz with a V.sub.osc of 15 millivolts, and a leakage current of 7.84.times.10.sup.-8 amps/cm.sup.2 at a field of 200 KV/cm has been made in thin films of less than 2000 .ANG., which are suitable for use in integrated circuits.
The development of a process that consistently produces high quality thin films has lead to the discovery that a class of materials, called layered superlattice materials herein, have remarkable properties. Layered superlattice materials having high polarizabilities, extremely low fatigue when switched over long periods in integrated circuits, high dielectric constants, and many other previously unknown electrical properties have been fabricated. Now that process of making high quality layered superlattice materials and reliable integrated circuits utilizing the layered superlattice materials has been discovered, this process can be applied to creating numerous other integrated circuits and other devices utilizing these materials and many other related materials.